Osteogenic fusion device

ABSTRACT

An interbody osteogenic fusion device is provided that includes opposite end pieces with an integral central element. The end pieces are sized to maintain the height of an intervertebral disc space. The central element has a much smaller diameter so that the osteogenic fusion device forms an annular pocket around the central element. An osteogenic material is disposed within the annular pocket between the opposite end pieces. In one embodiment, the osteogenic material constitutes a collagen sheet soaked in a solution containing a bone morphogenetic protein. The osteogenic fusion device is configured so that the osteogenic material is in direct contact with the adjacent vertebral bone. In addition to the enhanced area of contact between the vertebral bone and the fusion material, the inventive osteogenic fusion device reduces stress-shielding and minimizes the radio-opacity of the implant so that growth of the fusion mass can be continuously assessed.

BACKGROUND OF THE INVENTION

The present invention relates to an implant to be placed into theintervertebral space left after the removal of a damaged spinal disc.Specifically, the invention concerns an osteogenic fusion device thatenhances arthrodesis or fusion between adjacent vertebrae while alsomaintaining the normal spinal anatomy at the instrumented vertebrallevel.

In many cases, low back pain originates from damages or defects in thespinal disc between adjacent vertebrae. The disc can be herniated or canbe affected by a variety of degenerative conditions. In many cases,these pathologies affecting the spinal disc can disrupt its normalanatomical function of the disc. In some cases, this disruption issignificant enough that surgical intervention is indicated.

In one such surgical treatment, the affected disc is essentially removedand the adjacent vertebrae are fused together. In this treatment, adiscectomy procedure is conducted to remove the disc nucleus whileretaining the annulus. Since the disc material has been removed, a bodymust be placed within the intervertebral space to prevent the space fromcollapsing.

In early spinal fusion techniques, bone material, or bone osteogenicfusion devices, were simply disposed between adjacent vertebrae,typically at the posterior aspect of the vertebrae. In the early historyof these osteogenic fusion devices, the osteogenic fusion devices wereformed of cortical-cancellous bone which was not strong enough tosupport the weight of the spinal column at the instrumented level.Consequently, the spine was stabilized by way of a plate or a rodspanning the affected vertebrae. With this technique, once fusionoccurred across and incorporating the bone osteogenic fusion device, thehardware used to maintain the stability of the spine became superfluous.

Following the successes of the early fusion techniques, focus wasdirected to modifying the device placed within the intervertebral space.Attention was then turned to implants, or interbody fusion devices, thatcould be interposed between the adjacent vertebrae, maintain thestability of the disc interspace, and still permit fusion orarthrodesis. These interbody fusion devices have taken many forms. Forexample, one prevalent form is a cylindrical hollow implant or "cage".The outer wall of the cage creates an interior space within thecylindrical implant that is filled with bone chips, for example, orother bone growth-inducing material. Implants of this type arerepresented by the patents to Bagby, U.S. Pat. No. 4,501,269; Brantigan,U.S. Pat. No. 4,878,915; Ray, U.S. Pat. No. 4,961,740; and Michelson,U.S. Pat. No. 5,015,247. In some cases, the cylindrical implantsincluded a threaded exterior to permit threaded insertion into a tappedbore formed in the adjacent vertebrae. Alternatively, some fusionimplants have been designed to be impacted into the intradiscal space.

Experience over the last several years with these interbody fusiondevices has demonstrated the efficacy of these implants in yielding asolid fusion. Variations in the design of the implants have accountedfor improvements in stabilizing the motion segment while fusion occurs.Nevertheless, some of the interbody fusion devices still have difficultyin achieving a complete fusion, at least without the aid of someadditional stabilizing device, such as a rod or plate. Moreover, some ofthe devices are not structurally strong enough to support the heavyloads and bending moments applied at certain levels of the spine, namelythose in the lumbar spine.

Even with devices that do not have these difficulties, other lessdesirable characteristics exist. Recent studies have suggested that theinterbody fusion implant devices, or cages as they are frequentlycalled, lead to stress-shielding of the bone within the cage. It is wellknown that bone growth is enhanced by stressing or loading the bonematerial. The stress-shielding phenomenon relieves some or all of theload applied to the material to be fused, which can greatly increase thetime for complete bone growth, or disturb the quality and density of theultimately formed fusion mass. In some instances, stress-shielding cancause the bone chips or fusion mass contained within the fusion cage toresorb or evolve into fibrous tissue rather than into a bony fusionmass.

A further difficulty encountered with many fusion implants is that thematerial of the implant is not radiolucent. Most fusion cages are formedof metal, such as stainless steel, titanium or porous tantalum. Themetal of the cage shows up prominently in any radiograph (x-ray) or CTscan. Since most fusion devices completely surround and contain the bonegraft material housed within the cage, the developing fusion mass withinthe metal cage between the adjacent vertebrae cannot be seen undertraditional radiographic visualizing techniques and only with thepresence of image scatter with CT scans. Thus, the spinal surgeon doesnot have a means to determine the progress of the fusion, and in somecases cannot ascertain whether the fusion was complete and successful.

The field of spinal fusion can be benefited by an intervertebral fusiondevice that can support bone growth material within the intervertebralspace, while still maintaining the normal height of the disc space. Thedevice would beneficially eliminate the risk of stress-shielding thefusion mass, and would also provide for visualization of the fusion massas the arthrodesis progresses.

SUMMARY OF INVENTION

To address the current needs with respect to interbody fusion devices,the present invention contemplates a osteogenic fusion device that isconfigured to place as much of the bone growth inducing material aspossible into direct contact with the adjacent bone. In one embodiment,the osteogenic fusion device includes an elongated body having oppositefirst and second end pieces separated by an integral central element.The central element has a significantly smaller diameter than the twoend pieces. The osteogenic fusion device thus forms an annular pocketbetween the end pieces and around the central element.

In accordance with one aspect of the present invention, a bone growthinducing material is disposed within the annular pocket around thecentral element of the osteogenic fusion device. In one specificembodiment, the bone growth inducing material can constitute a sheet ofa pharmaceutically suitable carrier for a bone growth factor, such as abone morphogenetic protein. In this embodiment, the sheet can be acollagen sheet that is soaked with the BMP and then subsequently wrappedin spiral fashion around the central element of the osteogenic fusiondevice.

In one feature of the present invention, the osteogenic fusion devicecan be implanted in a bi-lateral approach. Specifically, two suchosteogenic fusion devices can be inserted into prepared bores formed inthe endplates of adjacent vertebrae after completion of a discectomy.The spinal loads are borne by the two end pieces that are in directcontact with the adjacent vertebral bodies. Preferably, the osteogenicfusion device has a length sufficient to allow the end pieces to atleast partially contact the harder bone at the apophysis of the adjacentvertebrae. With the osteogenic fusion device thus inserted, the bonegrowth inducing material is in direct contact with the adjacentvertebral bodies. In addition, bone growth inducing material can beplaced within the bi-lateral space separating the two osteogenic fusiondevices. When fusion occurs, a substantial fusion mass is produced thatis virtually uninterrupted by the material of the osteogenic fusiondevice itself.

Several alternative embodiments of the osteogenic fusion device arepresented, all retaining the capability of supporting bone growthinducing material so that it is in direct contact with the adjacentvertebrae. In some embodiments, additional elements of the centralelement are provided, while in another embodiment, an intermediate pieceis provided for further support across the disc space.

The present invention also contemplates an insertion tool and certainmodifications to the osteogenic fusion device to accommodate the tool.In one preferred embodiment, the tool is essentially an elongated shankhaving opposite prongs extending therefrom. The prongs can engagetruncated side walls of one of the end pieces. In addition, the oppositeend piece can be formed with notches to receive the tips of the twoprongs. With this design, the osteogenic fusion device can be a push-inor a threaded type osteogenic fusion device.

It is one object of the present invention to provide an interbody fusiondevice that allows the greatest possible contact between the adjacentvertebrae and the bone growth inducing material supported by theosteogenic fusion device. It is a further object to provide such aosteogenic fusion device that is capable of supporting the loadsgenerated throughout the spine without stress-shielding developing bonewithin the osteogenic fusion device.

Another object of the invention is achieved by features that minimizethe radio-opacity of the device. This results in a benefit to thesurgeon of being able to more readily assess the progress of a spinalfusion.

Other objects and benefits of the present invention can be discernedfrom the following written description and accompanying figures.

DESCRIPTION OF THE FIGURES

FIG. 1 is a top elevational view of a osteogenic fusion device inaccordance with one embodiment of the present invention.

FIG. 2 is an end elevational view of one end of the osteogenic fusiondevice shown in FIG. 1.

FIG. 3 is a top elevational view of an alternative embodiment of theosteogenic fusion device utilizing exterior threads.

FIG. 4 is a top cross-sectional view of a osteogenic fusion device asshown in FIG. 1 with a bone growth inducing material supported by theosteogenic fusion device.

FIG. 5 is an cross-sectional view of the osteogenic fusion device andbone growth material shown in FIG. 4 taken along line 5--5 as viewed inthe direction of the arrows.

FIG. 6 is a plan view of a sheet for a bone growth inducing materialused with the osteogenic fusion device shown in FIG. 4.

FIG. 7 is an end elevational view of one end of a osteogenic fusiondevice, such as the osteogenic fusion device of FIG. 1, modified toinclude apertures.

FIG. 8 is an end elevational view of one end of a osteogenic fusiondevice, such as the osteogenic fusion device of FIG. 1, modified toinclude apertures.

FIG. 9 is a side, partially cross-sectional view of an intervertebraldisc space with a osteogenic fusion device according to FIG. 1 implantedbetween adjacent vertebrae.

FIG. 10 is a top elevational view of the superior aspect of theinstrumented vertebral level shown in FIG. 9, depicting bilateralplacement of osteogenic fusion devices according to the presentinvention.

FIG. 11 is a cross-sectional view of the instrumented vertebral segmentshown in FIG. 10, taken along line 10--10 as viewed in the direction ofthe arrows.

FIG. 12 is a top elevational view of a osteogenic fusion device, such asshown in FIG. 1, with features to permit insertion of the osteogenicfusion device.

FIG. 13 is an end elevational view of the osteogenic fusion device shownin FIG. 12.

FIG. 14 is a side elevational view of an insertion tool according to oneembodiment of the present invention.

FIG. 15 is a top elevational view of the insertion tool shown in FIG.14.

FIG. 16 is a top elevational view of a osteogenic fusion device forrestoring the lordotic angle between adjacent vertebrae according to afurther embodiment of the present invention.

FIG. 17 is a top elevational view of a osteogenic fusion deviceaccording to a further embodiment of the present invention.

FIG. 18 is a top elevational view of a osteogenic fusion deviceaccording a still further embodiment of the present invention.

FIG. 19 is an end elevational view of the osteogenic fusion device shownin FIG. 18.

FIG. 20 is a top elevational view of a osteogenic fusion deviceaccording to another embodiment of the present invention.

FIG. 21 is an end elevational view of the osteogenic fusion device shownin FIG. 20

FIG. 22 is a top elevational view of a osteogenic fusion deviceaccording to yet another embodiment of the present invention.

FIG. 23 is an end elevational view of the osteogenic fusion device shownin FIG. 22.

FIG. 24 is a top elevational view of a osteogenic fusion deviceaccording to a further embodiment of the present invention.

FIG. 25 is an end elevational view of the osteogenic fusion device shownin FIG. 24.

FIG. 26 is a top elevational view of a pair of fusion devices accordingto FIGS. 24-25 disposed in a bilateral configuration in the lumbarspine.

FIG. 27 is a top elevational view of a fusion device according to FIGS.24-25 disposed in the cervical spine.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of theinvention is thereby intended, such alterations and furthermodifications in the illustrated device, and such further applicationsof the principles of the invention as illustrated therein beingcontemplated as would normally occur to one skilled in the art to whichthe invention relates.

The present invention contemplates osteogenic fusion devices for use asinterbody fusion devices. The osteogenic fusion devices include oppositeend pieces that are configured to span the intervertebral disc space andengage the adjacent vertebral bodies. The inventive osteogenic fusiondevices include a central element separating the two end pieces andsubstantially spanning the anterior-posterior length of the disc space.The invention further contemplates that a bone growth-inducing materialbe disposed about the central element and between the opposite endpieces. When the inventive osteogenic fusion device is implanted withina patient, the bone growth-inducing material is in direct contact withthe adjacent vertebral bodies. The end pieces are formed of a materialsufficient to withstand the spinal loads generated at the instrumentedvertebral level.

In accordance with one embodiment of the invention, an osteogenic fusiondevice 10, depicted in FIGS. 1-2, includes a first end piece 11 and asecond end piece 12. The end pieces are separated by a central element13. The first end piece 11 could be substantially cylindrical or anygeometrical shape and includes an outer bone contacting surface 15. Theend piece 11 also defines an inwardly facing retaining surface 17. Thecentral element 13 integrally extends from the retaining surface 17 ofthe first end piece 11.

The second end piece 12 also defines a bone contacting surface 20 that,in this embodiment, does not extend entirely around the end piece. Thebone contacting surface 20 could be any geometrical shape, preferablycircular and is defined at a radius equal to the radius of the outersurface 15 of the first end piece. Thus, as depicted in FIG. 2, the bonecontacting surface 20 of the second end piece 12 is generally coincidentwith portions of the outer surface 15 of the first end piece 11 when theosteogenic fusion device is viewed along the longitudinal axis of itscentral element 13. The second end piece 12 also includes oppositetruncated surfaces 21 that are disposed between the circular bonecontacting surfaces 20. Preferably, the truncated surfaces 21 aregenerally flat and can be configured to be engaged by an insertion tool.The insertion tool preferably has arms that contact the flat truncatedsurfaces 21, yet still fall within the envelope defined by the outersurface 15 of the first end piece 11.

The second end piece 12 also defines a second retaining surface 22 thatfaces the first retaining surface 17 of the first end piece 11. Again,the central element 13 is preferably integral with and projectsoutwardly from the second retaining surface 22. Alternatively, thecentral element can be in the form of a central rod that is engagedwithin colinear bores formed in the two end pieces. In this variation,the engagement between the central rod and the end pieces can bethreaded.

The central element 13 includes an outer central surface 23. Preferably,the central element 13 is substantially cylindrical along its length. Inone aspect of the invention, the first end piece 11 defines a diameterD₁, while the central element 13 defines a diameter D₂. The diameter D₁is at least equal to the height of the intervertebral space within whichthe osteogenic fusion device 10 is to be interposed. Most preferably,the diameter D₁ corresponds to the diameter of a cylindrical channel cutinto the endplates of the adjacent vertebrae. In this instance, thediameter D₁ will be somewhat larger than the intervertebral disc spaceheight. Moreover, the diameter D₁ is significantly larger than thediameter D₂ of the central element 13. This diameter differentialcreates an annular pocket 24 surrounding the central element 13.

The osteogenic fusion device 10 has a length L₁ between the oppositeends of the osteogenic fusion device. This length L₁ is preferablyselected to be slightly less than the anterior-posterior length of theintervertebral disc space, although the length can be calibrated to thelateral dimension of the space. Most preferably, the length L₁ is sizedso that the first and second end pieces 11, 12 can contact at least aportion of the apophysis or harder cortical bone at the perimeter of thevertebral endplates. The osteogenic fusion device 10 further defines alength L₂ which is essentially the length of the central element 13. Thelength L₂ is calibrated so that the end pieces 11 and 12 aresufficiently wide to provide adequate support between the adjacentvertebrae. Conversely, the length L₂ is sufficiently long so that theannular pocket 24 has the capacity for retaining a substantial quantityof bone growth-inducing material.

In a modification of the osteogenic fusion device 10, the second endpiece can be configured with threads. For example, as depicted in FIG. 3an end piece 25 includes external bone engaging threads 26 extendingfrom the outer surface 27. In accordance with this embodiment, thesecond end piece 25 can be cylindrical, like the first end piece 11, orthe threads can be formed between truncated surfaces, such as truncatedsurfaces 21 in the prior embodiment. At any rate, the threaded end piece25 is configured to be threadedly advanced into a drilled and tappedchannel within the adjacent vertebral bodies. The first end piece 11 canalso be threaded to facilitate insertion and to reduce the chance ofexpulsion.

In a further aspect of the invention, a bone growth inducing material 30is provided for support by the osteogenic fusion device 10. Preferablythe material 30 is in the form of a sheet. In a specific example, thecarrier sheet 30 can be a collagen sheet that is soaked with a solutioncontaining a bone growth inducing substance, or a bone morphogeneticprotein (BMP). In accordance with the invention, the carrier sheet 30can be formed of a variety of materials other than collagen, providedthe materials are capable of containing a therapeutically effectivequantity of a bone growth inducing substance or BMP. Moreover, thematerial 30, whether in sheet form or not, is most preferablysusceptible to manipulation to be disposed within the annular pocket 24of the fusion device 10.

In accordance with the specific embodiment, the carrier sheet 30 iswound around the outer surface 23 of the central element 13 (see FIG.5). The carrier sheet 30 is held between the retaining surface 17 of thefirst end piece 11 and the retaining surface 22 of the second end piece12. In accordance with one specific embodiment, the retaining surface 22is curved or convex. In this way, the carrier sheet 30 can project intothe convexity to serve as a sort of anchor to hold the carrier sheet 30within the annular pocket 24 of the osteogenic fusion device 10. Inaddition, the convex surface 22 conforms better with the anteriorportion of the vertebral body profile when the fusion device isimplanted.

In the illustrated embodiment, the carrier sheet 30 can be provided as asingle sheet, as shown in FIG. 6. The inner end 31 of the sheet isdisposed against the central outer surface 23 of the central element 13.The sheet can be wound in a spiral fashion about the central element 13until its outer end 32 is disposed adjacent the outer surface 15 of thefirst end piece 11. The carrier sheet 30 has width W that is preferablyslightly larger than the length L₂ between the first and second endpieces to allow a portion of the carrier sheet 30 to project into theconcave retaining surface 22 of the second end piece 12. The overalllength of the sheet 30 between ends 31 and 32 depends upon its thicknessand the difference in diameters D₁ and D₂. For example, in oneembodiment the diameter D₂ is about one-fourth (1/4) the diameter D₁.Preferably, the length is sufficient so that the carrier sheet 30 can betightly wound about the central element 13 and fill the annular pocket24. One important object of the present invention is that the carriersheet 30 or bone growth inducing material reside in direct contact withthe adjacent vertebral bone. Consequently, the sheet 30 is preferablywound so that its outer end 32 is at least slightly outside the envelopeof the outer surface 15 of the first end piece 11.

The carrier sheet 30 of FIGS. 4-6 illustrates one specific embodiment ofbone growth-inducing material usable with the osteogenic fusion deviceof the present invention. It is also contemplated that the carrier canbe in the form of a sponge, paste, gel or a settable osteogenicmaterial. The osteogenic material must be provided in some form that canbe generally retained about the central element 13 and within theannular pocket 24 of the osteogenic fusion device 10. Put differently,the present invention contemplates an osteogenic material that does notneed to be contained in the traditional manner of the hollow cylindricalcages of the prior art. In these prior art devices, cancellous bonechips have been contained within a hollow cage. The present inventiondoes not contemplate the use of bone chips alone. However, bone chipscontained within a bone paste or a gel may be utilized with theosteogenic fusion device 10, provided that the paste or gel have aconsistency sufficient to hold the bone growth inducing material on andwithin the osteogenic fusion device 10.

In accordance with one specific embodiment, the end pieces 11 and 12 aresolid and circular in configuration. Alternative end piececonfigurations are shown in FIGS. 7 and 8. For example, end piece 11'can have a plurality of generally circular apertures 34 disposedcircumferentially about the end piece, as shown in FIG. 7. The end piece11" shown in FIG. 8 includes a plurality of pie-shaped apertures 35 sothat the end piece gives the appearance of a spoked wheel. The secondend piece 12 of the osteogenic fusion device 10 can have similarapertures defined therethrough. The apertures 34 and 35 in the endpieces 11', 11" provide a further avenue for facilitating fusion bonegrowth. The apertures themselves can be filled with a osteogenicmaterial, such as a gel or a paste. Moreover, once the osteogenic fusiondevice 10 is implanted within an intervertebral disc space, osteogenicmaterial can be packed around the osteogenic fusion device within thedisc space. These additional apertures in the end pieces 11, 12 providefurther avenues for the formation of a bony bridge between adjacentvertebrae.

The end pieces 11,12, etc. can also have non-circular shapes. Forinstance, the end pieces can be rectangular or other multi-sided shapes.If the osteogenic fusion device resides within a channel prepared in theendplates, the channel shape can be modified to conform to the boneengaging surfaces 15, 20 of the end pieces.

FIGS. 9-11 depict a pair of osteogenic fusion devices 10 implanted in abi-lateral configuration between adjacent vertebral bodies V₁ and V₂. Asdepicted, the disc annulus A is retained but at least one portal must bedefined in the annulus A to permit insertion of the osteogenic fusiondevices 10. The present invention also contemplates insertion of eachosteogenic fusion device 10 through its own portal formed in the discannulus A. Alternatively, in conformance with other known procedures, asingle portal can be provided through which each osteogenic fusiondevice 10 is successively inserted. Further in accordance with thepresent invention, the osteogenic fusion devices 10 can be positionedwithin the intervertebral disc space according to known posterior orpostero-lateral techniques.

According to the present invention, the osteogenic fusion device 10 isinserted into the disc space S with the first end piece 11 proceedingfirst into the space. Preferably, a channel C is bored into thevertebral endplates E to a preferred depth of insertion of theosteogenic fusion device 10, in accordance with known techniques. If theosteogenic fusion device to be implanted is of the type shown in FIG. 3with the threaded second end piece 25, the channels C can beappropriately drilled and tapped to accommodate the bone engagingthreads 26. In a modification of this embodiment, the first end piece 11can also carry external threads.

The preferred embodiment contemplates a generally cylindrical osteogenicfusion device placed within circular channels. Alternatively, theosteogenic fusion devices can operate as spacers that directly contactthe endplates, without a prepared channel. In this instance, the boneengaging surfaces of the end pieces can be modified to conform to thevertebral endplate geometry.

As depicted in FIGS. 9-11, the osteogenic material 30 is disposed indirect contact with the adjacent vertebral endplates E. Moreover, theplacement of osteogenic fusion devices 10 can present a medial space 37between the two osteogenic fusion devices. Osteogenic material can thenbe placed within the medial space 37, again in direct contact with theosteogenic material 30 situated around the central elements 13 of eachof the osteogenic fusion devices 10. Once complete fusion occurs, newbone growth will substitute the carrier material 30 to form a solid bonybridge spanning the adjacent vertebrae V₁, V₂. As can be seen from FIGS.9-11, the region of continuous bone growth is very substantial and isnot interrupted by the structure of the fusion device itself.

It is understood, of course, that the end pieces 11 and 12 providesufficient support for the vertebral loads passing between the adjacentvertebrae. At the same time, this load bearing capacity is concentratedoutside the middle regions of the vertebral endplates E. It is knownthat the central region of the endplates is very rich in blood flow andhas a high capacity for new bone growth. Thus, the elimination ofstructural material of the osteogenic fusion device 10 from that regionprovides for a faster and more complete arthrodesis than may have beenpossible with prior fusion cages.

Referring next to FIGS. 14, 15, an insertion tool 50 is depicted forinserting a osteogenic fusion device 10 according to the presentinvention. The insertion tool 50 includes a solid shank 51 to which aknob or handle 52 is affixed. The knob 52 is configured for manualgrasping and manipulation during insertion of the osteogenic fusiondevice. In the case where the osteogenic fusion device is not threaded,the insertion tool 50 simply acts as a pushing device. On the otherhand, in the instance where the osteogenic fusion device includesthreaded end pieces such as shown in FIG. 3, the insertion tool 50 mustbe rotated as the end piece is threaded into the prepared channelbetween the adjacent endplates.

The insertion tool 50 includes a pair of prongs 53 that are disposedapart to define an end piece recess 54. For insertion of the osteogenicfusion device 10 shown in FIG. 1, the end piece recess 54 is configuredso that the prongs 53 are in tight contact with the truncated surfaces21 of the second end piece 12. The outer surface of the prongs 53 canconform to a portion of the outer surface 15 of the first end piece 11.

The insertion tool 50 depicted in FIGS. 14-15 also includes tapered tips55 at the ends of each of the prongs 53. These tapered tips areconfigured to be received within driving notches 41 in a modified firstend piece 40, as depicted in FIGS. 12-13. The osteogenic fusion devicedepicted in FIGS. 12-13 is substantially similar to the osteogenicfusion device 10 shown in FIG. 1, with the exception of the addeddriving notches. The insertion tool 50 is configured so that the tips 55project into the notches 41 while the prongs 53 directly contact thetruncated surfaces 21 of the second end piece 12. This particularconfiguration of the insertion tool is particularly useful for threadedinsertion of the osteogenic fusion device. Preferably, the prongs 53have an effective outer diameter that is approximately equal to thediameter D₁. Moreover, the prongs 53 can have an arc segmentconfiguration to complement the truncated surfaces 21. If the end piece12 is threaded (see FIG. 3), the prongs 53 can include complementarythreads along their length.

The present invention also contemplates a osteogenic fusion device forrestoring the normal lordotic angle of an intervertebral segment.Specifically, a lordotic osteogenic fusion device 60 includes a firstend piece 61 and a second end piece 62 as shown in FIG. 16. As with theprior embodiments, a central element 63 is provided to connect the twoend pieces. The outer surface 65 of the first end piece 61 is in theform of a frusto-conical surface. The outer surface 65 tapers toward thesecond end piece 62 at a preferred lordotic angle. Similarly, the outersurface 66 of the second end piece 62 is also tapered at a similarlordotic angle. Alternatively, the second end piece 62 can includethreads formed on the outer surface 66. While the threads 66 at thesmaller second end piece 62 may not contact the vertebral endplates atthe larger insertion end, the threads will contact the endplates at theanterior end of the intradiscal space and will act as an anchor toresist expulsion of the lordotic osteogenic fusion device 60.

The present invention contemplates several modifications to the basicosteogenic fusion device 10. For example, the osteogenic fusion device70 shown in FIG. 17 includes first and second end pieces 71, 72 and acenter piece 73 disposed between the two end pieces. First and secondcentral elements 74 and 75 connect each of the end pieces 71, 72 to thecenter piece 73. In this instance, the center piece 73 will contact theinterior of the disc endplates E. Osteogenic material, such as carriersheets 30, can be disposed or wound around each of the central elements74, 75 until the end of the bone growth inducing material is exposed atthe outer surface of the osteogenic fusion device 70.

In a further modification, a osteogenic fusion device 80 depicted inFIG. 18 includes first and second end pieces 81 and 82 that areconnected by a plurality of central beams 83. In the illustratedembodiment as shown in FIG. 19, four such beams 83 are provided;however, other arrangements and numbers of beams are contemplated.Important aspects of the present invention are retained by theosteogenic fusion device 80 because osteogenic material can be supportedby the several beams 83 between the first and second end pieces 81, 82,with the bone growth inducing material in direct contact with theadjacent vertebral bodies.

The two embodiments of FIGS. 20-21 and FIGS. 22-23 pose a slightdeviation from the general concept of the osteogenic fusion device 10.In these two embodiments, the smaller diameter central element 13 isreplaced by a wall. In the embodiment of FIGS. 20-21, a osteogenicfusion device 85 includes first and second ends 86, 87 separated by acentral element 88. The first and second ends 86 and 87 can be in theform of short cylindrical sections, such as the first end piece 11 ofthe osteogenic fusion device 10 in FIG. 1. While the central element 88can be in the form of a solid wall, the osteogenic fusion device 85preferably includes a number of slots 89 defined through the centralelement 88. In accordance with the specific embodiment, the slots extendalong substantially the entire length of the central element 88. Whilethe osteogenic fusion device 85 deviates somewhat from the concept ofthe osteogenic fusion device 10, this latter osteogenic fusion device 85retains the broad beneficial feature of the present invention, namelyprovision for direct contact between osteogenic material supported bythe osteogenic fusion device 85 and the vertebral endplates. In thepresent instance, the osteogenic material can be situated on oppositesides of the central element 88. In addition, the material can be passedthrough the slots 89.

Preferably, the osteogenic fusion device 85 will be oriented within theintervertebral disc space with the central element 88, or wall, spanningbetween the adjacent vertebrae. This central element 88, then, willprovide additional structure and load bearing capability for sustainingthe spinal loads at the instrumented level.

The osteogenic fusion device 90 of FIGS. 22-23 operates on a similarconcept to the osteogenic fusion device 85. However, in this instance,the first and second end pieces are in the form of arc segments, ratherthan shortened cylinders. Specifically, the osteogenic fusion device 90includes upper and lower first arc segments 91_(U) and 91_(L), and upperand lower second arc segments 92_(U) and 92_(L). The osteogenic fusiondevice 90 also includes a central element 93 that is again in the formof a wall connecting the first and second end pieces. As can be seenmost clearly in FIG. 23, the arc segments 91, 92 and central element 93define a pair of cavities 96 for containing osteogenic material. In thisembodiment, the osteogenic material can be contained completely from endto end of the osteogenic fusion device 90. In the prior embodiments, theosteogenic material is contained within retaining surfaces of theopposite end pieces. In accordance with a specific embodiment, theosteogenic fusion device 90 includes a plurality of apertures 94 definedin each of the upper and lower first and second arc segments 91_(U),91_(L), 92_(U) and 92_(L). Similarly, a plurality of apertures 95 can bedefined through the central element 93. In this manner, the aperturesprovide the maximum capacity for bone ingrowth not only around, but alsothrough the osteogenic fusion device 90.

A osteogenic fusion device 100 shown in FIGS. 24-25 again presents aslightly different concept. This osteogenic fusion device 100 includes afirst end plate 101, a second end plate 102 and a central element 103that are similar to the like-named components of the osteogenic fusiondevice 10. However, the osteogenic fusion device 100 also includes aside piece 104 spanning between the first and second end pieces 101,102. Moreover, unlike the osteogenic fusion device 10, the first andsecond end pieces 101, 102 are not generally circular in configuration,but are generally rectangular in configuration. In one specificembodiment, the end pieces 101, 102 can include cut outs 105 at oppositesides of the end pieces to provide further avenues for the formation ofa bony bridge between adjacent vertebrae. As with the prior embodiments,the osteogenic fusion device 100 provides means for adequatelycontaining osteogenic material, such as in the form of the carrier sheet30. In this embodiment, the carrier sheet 30 can be wound around thecentral element 103, in the manner described above. This particularembodiment of the invention, namely osteogenic fusion device 100, ispreferably adapted for use in the lumbar spine as illustrated in FIG. 26and in the cervical spine illustrated in FIG. 27, and is consequentlysized accordingly.

The present invention contemplates osteogenic fusion devices that areformed of a material that is sufficiently strong to support the adjacentvertebrae and to maintain the disc height of the instrumentedintervertebral space. For example, the osteogenic fusion devices, suchas osteogenic fusion device 10, can be formed of a biocompatiblesterilizable metal, such as stainless steel or titanium. Of course,other medical grade materials are contemplated, such as certainceramics, polymers, etc., as well as allograft and xenograft bone,provided the materials are sufficiently strong. The overall dimensionsof each of the osteogenic fusion devices described above depends uponthe instrumented level. For example, a osteogenic fusion device for usein the cervical spine must necessarily be smaller than a osteogenicfusion device used in the lumbar spine. Moreover, the relativedimensions of the components of the osteogenic fusion devices may bealtered depending upon the vertebral level to be instrumented. Forexample, a osteogenic fusion device, such as osteogenic fusion device10, for use in the lumbar spine, may require a central element 13 havinga diameter D₂ that is more than one fourth of the outer diameter D₁ ofthe outer surface 15 of the first end piece 11. In some instances, thelumbar spine may generate bending moments across a osteogenic fusiondevice, such as osteogenic fusion device 10, that would require astronger central element 13.

In accordance with the present invention, the illustrated osteogenicfusion devices can be of the push-in or threaded-in type. Of course, theend pieces, such as end pieces 11, 12 of osteogenic fusion device 10,can include various surface characteristics known in the art forenhancing the degree of fixation of the osteogenic fusion device betweenthe adjacent vertebrae. For example, the end pieces can include certainmacro surface features for penetrating the vertebral endplates to resistexpulsion of the osteogenic fusion devices. Likewise, the surfaces, suchas outer surface 15 and bone contacting surface 20 can be provided withbone ingrowth coatings so that a certain amount of bone ingrowth occurseven between the end pieces and the adjacent vertebral bodies.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same is to be considered asillustrative and not restrictive in character, it being understood thatonly the preferred embodiments have been shown and described and thatall changes and modifications that come within the spirit of theinvention are desired to be protected.

What is claimed is:
 1. An implant for promoting fusion bone growth inthe space between upper and lower adjacent vertebrae, comprising:a loadbearing member including opposite end pieces and a rigid elongatedcentral element extending between said end pieces, said opposite endpieces each having two opposite surfaces configured to contact andsupport the upper and lower adjacent vertebrae, said opposite end piecessized to maintain the space between the adjacent vertebrae, said centralelement being sized relative to said opposite end pieces to define apocket between said central element and the adjacent vertebrae when theadjacent vertebrae are supported by said opposite end pieces, saidpocket configured to contain an osteogenic material disposed about saidcentral element and in intimate contact with the adjacent vertebrae whenthe vertebrae are supported by said opposite end pieces; and abiocompatible sheet carrier for an osteogenic material, saidbiocompatible sheet carrier wound around said central element withinsaid pocket.
 2. The implant for promoting fusion bone growth accordingto claim 1, wherein said opposite end pieces include:a first end piecehaving a first dimension between said two opposite surfaces, said firstdimension being sized to maintain the space between the adjacentvertebrae; and a second end piece having a dimension between said twoopposite surfaces substantially equal to said first dimension and asecond dimension transverse to said dimension that is less then saidfirst dimension.
 3. The implant for promoting fusion bone growthaccording to claim 1, wherein each of said two opposite surfaces of eachof said opposite end pieces is arcuate.
 4. The implant for promotingfusion bone growth according to claim 3, wherein said opposite endpieces include:a substantially circular first end piece; and a secondend piece with said two opposite surfaces being circular and includingtruncated non-circular sides between said two opposite surfaces.
 5. Theimplant for promoting fusion bone growth according to claim 1,wherein:each of said opposite end pieces defines a first dimensionbetween said two opposite surfaces; and said elongated central elementdefines a central dimension transverse to said longitudinal axis that isless than said first dimension.
 6. The implant for promoting fusion bonegrowth according to claim 5, wherein said central dimension is no morethan about 25% (twenty-five percent) of said first dimension.
 7. Theimplant for promoting fusion bone growth according to claim 1, furthercomprising an osteogenic material contained within said pocket andarranged to contact the adjacent vertebrae when the vertebrae aresupported by said opposite end pieces.
 8. The implant for promotingfusion bone growth according to claim 7, wherein said osteogenicmaterial includes an osteogenic substance disposed within saidbiocompatible sheet carrier.
 9. The implant for promoting fusion bonegrowth according to claim 1, further comprising an osteogenic materialcontained within said pocket and arranged to contact the adjacentvertebrae when the vertebrae are supported by said opposite end pieces.10. The implant for promoting fusion bone growth according to claim 8,wherein said osteogenic substance is a bone morphogenetic protein. 11.The implant for promoting fusion bone growth according to claim 1,wherein said two opposite surfaces of at least one of said end piecesincludes threads.
 12. The implant for promoting fusion bone growthaccording to claim 1, wherein each of said opposite end pieces defines aretaining surface connected to said central element, at least oneretaining surface being concave.
 13. The implant for promoting fusionbone growth according to claim 1, wherein at least one of said oppositeend pieces includes a plurality of apertures defined therethrough incommunication with said pocket.
 14. The implant for promoting fusionbone growth according to claim 1, wherein said central element isattached to said opposite end pieces substantially equidistant from eachof said two opposite surfaces of each end piece.
 15. The implant forpromoting fusion bone growth according to claim 1, wherein said loadbearing member includes a center piece having two opposite surfacesconfigured to contact the adjacent vertebrae, said center piece beingconnected to said central element between said opposite end pieces andbisecting said pocket.
 16. The implant for promoting fusion bone growthaccording to claim 1, wherein said central element is attached to saidopposite end pieces substantially equidistant from each of said twoopposite surfaces of each end piece.
 17. The implant for promotingfusion bone growth according to claim 1, further comprising a side piecespanning between said opposite end pieces and offset from said centralelement.
 18. The implant for promoting fusion bone growth according toclaim 1, wherein said central element includes at least two rodsconnected to said opposite end pieces.
 19. The implant for promotingfusion bone growth according to claim 1, wherein said central elementincludes a wall connected to said opposite end pieces, said wallbifurcating said pocket.
 20. The implant for promoting fusion bonegrowth according to claim 19, wherein said wall includes at least oneopening defined therethrough and communicating with said pocket.
 21. Theimplant for promoting fusion bone growth according to claim 20, whereinsaid at least one opening is an elongated slot.
 22. The implant forpromoting fusion bone growth according to claim 1, wherein at least oneof said opposite end pieces includes opposite arc segments, said arcsegments defining said two opposite surfaces contacting the adjacentvertebrae.
 23. The implant for promoting fusion bone growth according toclaim 22, wherein said opposite arc segments include a plurality ofapertures defined therethrough communicating with the adjacent vertebraewhen said two opposite surfaces are in contact with the adjacentvertebrae.
 24. The implant for promoting fusion bone growth according toclaim 1, wherein said elongated central element has a length betweensaid opposite end pieces, said length being sized to maintain contactbetween said opposite end pieces and cortical bone of the adjacentvertebrae when said load bearing member is in contact with the adjacentvertebrae.
 25. An implant for promoting fusion bone growth in the spacebetween upper and lower adjacent vertebrae, comprising:a load bearingmember including opposite end pieces and a rigid elongated centralelement extending between said end pieces, said opposite end pieces eachhaving two opposite surfaces configured to contact and support the upperand lower adjacent vertebrae, said opposite end pieces sized to maintainthe space between the adjacent vertebrae, said opposite end pieces eachdefining a retaining surface connected to said central element, at leastone retaining surface being concave, said central element being sizedrelative to said opposite end pieces to define a pocket between saidcentral element and the adjacent vertebrae when the adjacent vertebraeare supported by said opposite end pieces, said pocket configured tocontain an osteogenic material disposed about said central element andin intimate contact with the adjacent vertebrae when the vertebrae aresupported by said opposite end pieces.